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When considering theories regarding the relationship between the mind and the brain, identity theory plays an important role in our investigation. The identity theory of mind is the idea that 'each and every mental state is identical with some state in the brain'. Unlike similar theories, the identity theory does not simply want to make the claim that mind states correlate with brain states, but instead, hopes to prove that the two are identical. According to this line of argument, my desire for a slice of chocolate cake, my thirst and my belief that the lights in the room have begun to flash all sorts of colours, are all states of my brain. Identity theorists, unlike dualists, believe there is an aggregation of the mental unified as one, in the form of a physical entity, this being the brain. Whilst the theory is favoured due to its predictions of the existence of mind-brain correlations without appealing to mind-independent realms, it does face some problems. Enquiring into difficulties such as multiple realizablity and Leibniz's law of indiscernible identicals, we shall determine to what extent the theory is capable of being saved by defending itself of such critiques.

In order to determine whether the identity theory can be plausibly defended, we must first outline the theory further. When concentrating on the identity theory we can distinguish between two kinds of identity; types and tokens. Type theorists are concerned with kinds of things and not specifics. For example, type identity theorists focus on cats and their experience of pain, whereas a token identity theorist would focus closer on each specific cat in question, like Simon's cat, Bruno. For the most part, we will be concerned with the type identity theory. Advocated by Smart and Place, the type identity theory is the idea that 'mental states are physical states of the brain'. Exploring type identity, Smart thought the best way to illustrate the idea was to highlight this through examples of objects and ideas bearing the same relationship. Brain states and mind states then, obtain a similar relationship to that of water and h2O, and lightning to atmospheric electrical charge. Analogies such as these are especially important for the theory, as they highlight the two identities are not merely lawfully correlated, but are to be valued as one and the same thing, They are to be understood as analogous, resulting in Smart advocating an ontological simplicity through the use of an Occam's razor approach to the mind brain relationship. Just like whenever there is lighting there is an electrical discharge, whenever there is a mind state, there is also a brain state. For example, whenever I experience pain, there is some C-fibers firing which make me experience such pain, It follows from such an example, we are to understand that the C-fibers firing and the pain experienced are to be understood as one phenomenon. Through this law of parsimony, the extra entities which dualism appeals to are not needed, the theory need not appeal to anything non-material as the material states and processes alone are capable of this. If the theory can show that both mind and brain states are a part of a physical unity, it has succeeded. However, if it cannot show that mind states are material the way the brain states are, the theory becomes increasingly difficult to save.

The plausibility of the mind-brain identity theory becomes dubious when we begin to question the locations of certain types of feelings. Through Leibniz's law of indiscernible identicals, anything which is identical must be completely identical in its properties. For example if X and Y are identical, X's properties will be the same as Y's properties. If all of the properties of mind states are the same as the properties of the brain states then problems do not arise, however there has been many attempts to prove that because mind states have properties which differ to brain states, they cannot be identical. One example of this is to reason my pain is in my foot, however it is obvious that my C-fibers firing are not. If my pain has a location like the pain being in my foot, brain states and mind states cannot be identical. The pain in my foot would have a location distinctly different from that of my brain states or C-fibers which would be located in my brain. In response to this, the identity theorist believes we are wrong when we say we have a pain in our foot, instead, we should refer to this feeling as an 'in-the-foot' kind of pain with each separate location of pain in the body being referred to different brain state. To make clear, say we experience the pain in our foot, such a pain relates to the brain state B12, however, if we were to experience pain somewhere entirely different, like our elbow we would be experiencing a brain state b4. The pain is not actually located in your foot, but instead is a representation of pain found in your brain state. Such a reply seems appealing, as not only does it keep the identity between the mind and brain intact, but it also has scope for being actualised in cases concerning phantom pains. There are cases of people who have had their leg amputated, and following this, have claimed to still be able to experience pain in their leg, despite not having a leg. Such a reply and an example to make reference to, results in the theory plausibly defending itself my acknowledging location as a representation and not as an object or property.

Following this, Leibniz's Law of indiscernible identicals further adds to the problem of identity through identifying specific feelings of pains. Objecting to the identity theory one could raise the question 'my pain is sharp but what in my brain is sharp?'. In a similar way to the last questions reply, the identity theorist argues, to consider a pain sharp, is again, to be misunderstood with regards to realising pain, The person who takes this line of argument is understanding 'sharp' too literally. Instead, we are to understand such properties of pain as relating to different brain states. To add to this, Place wants to make clear the mistake we are making is when we are describing our experiences and how things smell, taste and feel, we are appealing to things in the phenomenal field, when in reality, we are undergoing an experience in the mind of something going on which is like what is going on when something has hurt me in a sharp way. To have a throbbing pain is to have a brain state relating to pains which feel as through they are throbbing whereas a sharp pain, is to have a separate brain state which is identified with only sharp pains, with sharp and throbbing used as concepts in order to describe the experience of the goings on when undergoing experiences. As a result, 'when we appear to attribute properties to the pain, we are attributing concepts to the pain experience', such experiences which are identical with brain states.

A further application of Leibniz's law proves the hardest fort he identity theory to defend. Colin McGinn considers qualia, our phenomenological experiences and mental states, such as having pain and feeling sick and how they feel to us. When we consider pains, we are aware they hurt, we state "i am in pain" or "it hurts". McGinn argues, 'how could technicolor consciousness arise from grey brain matter?". In other words, how can we experience pain consciously, if our brains are focussed and limited to the unconscious realm? We have consciousness of pain and its ability to hurt us, but no consciousness of what goes on in our own brains, Through this understanding it seems that pains have the property of hurting which brains do not. McGinn's argument shows that if pains have the property of hurting, it logically follows that pain cannot be identical to C-fibers firing on the basis that they do not have the property of hurting, As a result, with regards to Leibniz's law, the identity theory plausibly defends itself from two out of three problems posed, however McGinn's criticism proves difficult to rebuff.
Leibniz's law of indiscernibles is not the only problem the identity theory faces. Asymmetry in relation to first and third person access to mental states poses many difficulties. We understand ourselves to have privileged first person access to our own independent thoughts and sensory experiences, while everyone else has third person access to our mental life. In other worlds, 'I infer your thoughts and feelings by observing your circumstances, demeanour and behaviour', I have no way of actually knowing them unless you make them apparent to me. If we were to observe brain happenings, I amy be able to comprehend what you may be feeling or thinking, but my access of such thoughts and feelings differs greatly from yours. Instead of appealing to private thought chambers like the dualist does, Place urges us to consider access of the brain like that of a doctor's observations of your stomach. In contrast, our knowledge of our mental life is unmeditated, we do not need observe or recognise our own feelings or thoughts through any further inward investigation, these mind states are just self-revealing to us. As a result, Place highlights how it is reasonable to state, as the theory does, that there can remain a certain asymmetry to mind and brain states, yet the two remain identical.

When considering the identity theory, some have opted for an adapted version of the theory, known as restricted identity theory. Such a theory restricts identities of mind and body dependent species in question and is concerned with multiple realizability. However, the adaptation too, is not without criticism. Animals of various sorts, undergo and realise pains in a multitude of ways. in humans we may suppose our pain is realised through the C-fibers firing. However, when we consider different sorts of animals, we realise they are neurologically and biologically different to us, therefore their nervous systems will also be dramatically different to ours. This raises serious problems as Hilary Putnam notes. We have already established that the identity theory holds the view that mind states and brain states are identical however if pain equals C-fibers firing in me, by my fish do not have C-fibers, yet still experience pain, then the concept of pain and C-fibers firing cannot be identical. The identity theory notes this and attempts to keep the theory plausible by amending it accordingly. Instead of generalising that C-fibers firing are equal to all pains, we should restrict the theory to what pain is equal to, depending on the species we are referring to. For example, perhaps the pain of fish is recognised through Y-fibers, or the pain of snakes is recognised through T-fibers. Then we would arrive at a theory which identifies pain as follows: Pain in A equals X, pain in B equals Y and pain in C equals Z. With this in mind, multiple realizability proves a difficult critique for the identity theorist to answer. Following this, some type identity theorists have abandoned type altogether, opting for a token identity theory which allows for further introspection into specific, independent tokens of pains and not through the type identity theorists categorising and groupings of people's brain and mind states.

To conclude, whilst the mind and brain identity theory does have some considerable criticisms to answer to, more often that not, its replies to such problems are strong, allowing the theory to remain considerably plausible. One problem the theory is yet to fully answer is the question of multiple realizability. However with this being said, this should not mean we should eliminate the theory altogether, as with further discoveries in the scientific fields of neuroscience and biology we may very well be able to answer such a question helping us to further our understanding of the brain and the mind. In addition to this, whilst the theory was preceded by functionalism, it appears that if the two work together, with type identity injecting the concept of function into it's understanding of how C-fibers work, the theory could be considerably more plausible. An inclusion of functionalism would result in the identity theorist being able to defend themselves against criticisms concerning multiple realizablity, by showing that these realisations, although different in species still show to serve the same functions. Therefore whilst the identity theory remains plausible, if it were to combine the concept of functionalism into the equation it could have the potential to be almost infallible.